One method to control intake and exhaust valve operation during engine operation is described in U.S. Patent Publication No. U.S. 2003/0106515. This method presents a means to operate a variable event valvetrain during vehicle starting and stopping. The method attempts to control valve operation after a request to stop or start the engine has been made.
During a stop sequence, the method reduces valve lift and fuel in an attempt to gradually reduce engine speed without disturbing the driver. By controlling the inducted air amount and the fuel amount, the method attempts to control engine torque so that the engine will decelerate to a stop in a more controlled manner.
The method also attempts to control engine starting by adjusting valve lift. In one embodiment the valve lift is moved to a desired amount just after an engine is stopped and then is held constant during a subsequent engine start. In a second embodiment valve lift is adjusted to a predetermined position just before a start and then is held constant during the start. These valve lift strategies attempt to provide a smooth start when an engine is automatically started.
In addition, the method also attempts to reduce engine emissions after an engine is stopped by setting the valve lift to a predetermined position.
The above-mentioned method can also have several disadvantages. Specifically, the method controls valve timing and fuel amount during engine stopping without regard to combustion stability. By not recognizing that combustion stability may be affected by reducing cylinder air amount, the method may produce misfires and increase engine emissions.
In addition, vehicle emissions may be higher than desired during an engine start since the above-mentioned methods may allow oxygen to be pumped through the engine as the valves follow a constant lift command. In other words, the engine is started in a manner that is similar to an engine having a fixed valve lift amount and similar emissions may be expected. For example, during starting, engine position may not be known until the engine has rotated to a certain position. As a result, cylinder fueling may be delayed so that the cylinders may be fueled in a predetermined combustion order (e.g., 1-3-4-2 for a four cylinder engine). By delaying fueling and thus delaying combustion, air may be pumped through the engine to a catalyst, during at least a portion of the starting sequence. The air may cool the catalyst and may also supply oxygen to catalyst sites that may have otherwise reduced NOx. Consequently, lower catalyst temperatures and fewer reduction sites may decrease catalyst efficiency during an engine start.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method of controlling a variable event valvetrain that offers substantial improvements.